Why do functions inside a keywords list passed as arguments to `if` aren't called immediately?

Questions / Help
,
1

Why do functions inside the keywords list passed as arguments to if aren’t called immediately? Is this unique to if?

# example.exs
defmodule Call do
  def this(), do: IO.puts("this")
  def that(), do: IO.puts("that")
end

defmodule Example do
  def fun(keywords), do: :ok
end

IO.puts(
  Example.fun(
    [do: Call.this(), else: Call.that()]
  )
)

IO.puts(
  if(
    true,
    [do: Call.this(), else: Call.that()]
  )
)

Output from file:

$ elixir example.exs
this
that
ok
this # expected 'this' and 'that' always to be called independently of the if the boolean evaluation
ok
2

if is not a function, it is a macro. Macros run at compile-time and receive the AST representation of their arguments without any evaluation.

The guide has a good example of implementing an if-like macro:

2 Likes
3

Thanks! :slight_smile:

4

If it was written like this, would you still expect both to be called?

IO.puts(
  if true do
    Call.this()
  else
    Call.that()
  end
)

I suppose not, because what would be the point of if then? But both of these codes are equivalent. In fact the latter is just syntax sugar for the former.

5

No, from the syntax sugar the logic flow is clear. My doubt came from reading Optional Syntax Sheet - Getting Started.

My question was about why the two codes work differently even though they are the same and not about how can I make sense of this. :slight_smile: